For a bottle 5oz and smaller, add 1 teaspoon of water. For bottles 6oz to 12oz, add 1 tablespoon of water. For bottles 12oz to 16 oz, add 2 tablespoons.
Keep an eye on your planter after you have assembled it. You want to see water condensing on the top of the bottle and wet soil the next day. If you do not see this after one day, add another teaspoon per day until you do.
Conversely if you see mold growing in your seed starter, cut back the water by one teaspoon the next time you try. (3 teaspoons = 1 tablespoon)